Concrete Base over Existing Foundations for Small Concrete Garage

[5 Bobs]

Hi I'm in the process of laying a 4 x 3 mtr slab to build a small concrete sectional garage on top of.
My initial plan has been to and remove 150mm of soil and then to refill with 100mm of hardcore and then sand, dpm and 100mm of concrete.
How ever once soil removed I have come across the foundations for the outside wall to our garden this runs the length of my base and about 30-40cm wide. My question is how do I include this into my base? If I just concrete over then it will mean at that point the new concrete will only be about 50mm thick over the old.
If this ok to do what do I do about the dpm? Laying it inbetween the old and new concrete doesn't sound right!
Many thanks for any help advice.
Ian

[RICKSGARAGE]

Hi, I see nobody replied to your question but my problem is similar. I want to extend my single skin 5mx5m block garage by about 11m2 ( 5x2.2m) so not unlike your size project.It will be a workshop and the car will not be parked on the new area. Did you find your 100mm hardcore, 50mm sand and 100mm concrete works. I'm thinking it should be more than enough but some talk about 750mm deep trench and 150mm reinforced slab as well.